⇦Chinese Physics Letters, 2017, Vol. 34, No. 7, Article code 077701 Abnormal Polarity Effects of Streamer Discharge in Propylene Carbonate under Microsecond Pulses * Hong-Wei Liu(刘宏伟)1, Yan-Pan Hou(侯炎磐)1, Zi-Cheng Zhang(张自成)1**, Jian Xu(徐健)2 Affiliations 1College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073 2Chongqing Communication Institute, Chongqing 400035 Received 30 December 2016 *Supported by the National Natural Science Foundation of China under Grant No 51677190, and the Hunan Provincial Natural Science Foundation of China under Grant No 2017JJ1005.
**Corresponding author. Email: zczhang@nudt.edu.cn Citation Text: Liu H W, Hou Y P, Zhang Z C and Xu J 2017 Chin. Phys. Lett. 34 077701 Abstract Propylene carbonate (PC) has a great potential to be used as an energy storage medium in the compact pulsed power sources due to its high dielectric constant and large resistivity. We investigate both the positive and negative breakdown characteristics of PC. The streamer patterns are obtained by ultra-high-speed cameras. The experimental results show that the positive breakdown voltage of PC is about 135% higher than the negative one, which is abnormal compared with the common liquid. The shape of the positive streamer is filamentary and branchy, while the negative streamer is tree-like and less branched. According to these experimental results, a charge layer structure model at the interface between the metal electrode and liquid is presented. It is suggested that the abnormal polarity effect basically arises from the electric field strength difference in the interface between both electrodes and PC. What is more, the recombination radiation and photoionization also play an important role in the whole discharge process. DOI:10.1088/0256-307X/34/7/077701 PACS:77.22.Jp, 52.80.Wq, 51.50.+v © 2017 Chinese Physics Society Article Text In recent years, with rapid development of pulsed power technology, the application domain of the pulsed power generator with high average power and compact structure is gradually broadened, which demands a miniaturized and light energy storage system. Liquid dielectrics are widely used as energy storage media in pulsed power systems because of their high breakdown strength, good heat dissipation ability and ease of handling.[1-6] Propylene carbonate (PC) has shown a promising application prospect in the pulsed power technology as a kind of high energy storage liquid dielectric due to its high dielectric constant, large resistivity and good temperature adaptability.[7] Kolb et al.[8] obtained that the breakdown strength of PC could be 2.2 MV/cm with the polished stainless steel electrodes, which was larger than that of water, and a maximum charge energy density of 14 J/cm$^{3}$ made it possible to be used as a high energy storage dielectric in a compact pulsed power device. The research of streamer structures (plasma arcs) and light emitted in different polarities is conducive to revealing the intrinsic characteristics of liquid breakdown. However, there is no perfect mechanism for explaining the initiating and propagation of streamer discharge.[9,10] Previous progress has shown that streamer formation and propagation have different structures and velocities depending on the polarity of the needle electrode during the breakdown process.[11] Qian et al.[12] built a simulation model of the electrical breakdown that emphasized the local and low-density field-emission processes. It was found that polarity effects resulted from the large difference in charge mobility between ions and electrons. Jones and Kunhardt proposed a bubble breakdown model in the microsecond or sub-microsecond order to explain the difference of streamer discharge due to variation in background pressure.[13] Moreover, it has been shown that the electrical breakdown characteristics of the liquid are related to not only the duration and polarity of the applied voltage, but also the electrode parameters such as material, geometry and the electrode gap, which is a considerably complicated process. In terms of the pulse duration of voltage ($t_{\rm br}$), the breakdown field $E_{\rm br}$ is inversely proportional to it, raised to a power $n$, i.e., $E_{\rm br}\sim t _{\rm br}^{-n}$, which is known as the volt-time breakdown characteristic.[14-19] In this work, the experimental research on the polarity effect of streamer discharge in PC was carried out by a compact pulsed power source charging on a needle-plate electrode system under microsecond pulse. To explore the abnormal polarity effect in PC, optical diagnostic techniques were used to record the development of the streamer. At last, a charge layer structure model was presented to explain the abnormal experimental results.
cpl-34-7-077701-fig1.png
Fig. 1. The experimental system for the electrical breakdown of PC.
The experimental system for the electrical breakdown of PC is shown in Fig. 1. The entire platform consists of a pulsed power source, a test cell with needle-plate electrodes and an ultra-high-speed camera. The power supply with the 220 V ac-voltage charges the primary capacitance after half-wave rectification. The pulse-forming line (PFL) is charged by the primary capacitor through the Tesla transformer when the main thyristor closes. When the voltage across the needle-plate electrode reaches the breakdown threshold of PC, the breakdown process occurs. The impulse voltage applies to the needle-plate electrode made of stainless steel producing a strong non-uniform electric field in PC between electrodes. According to the market demand, the pulse duration of voltage varied from 1 μs to 10 μs in the experiment. The inter-electrode distance of the needle-plate electrode system is 5 mm and the resistivity of PC is 222 M$\Omega$cm. The charging voltage applied to the primary capacitance was set to be 200 V. The breakdown voltages were measured with a standard Tektronix high-voltage probe (P6015A), and the waveforms were recorded by a Tektronix oscilloscope (TDS 3054). A synchronous trigger circuit for the ultra-high-speed camera and the pulsed power supply was used to record the streamer patterns.
cpl-34-7-077701-fig2.png
Fig. 2. Breakdown voltage waveforms in different polarities.
Typical experimental waveforms of the breakdown voltages in different polarities are shown in Fig. 2. Each breakdown case has been tested 20 times. Since the breakdown of insulation materials has a certain randomness, a two-dimensional Weibull distribution is adopted in data analysis, as shown in Fig. 3. In general, the breakdown voltage of the liquid dielectric is considered to be the value corresponding to the breakdown probability of 0.5. Experimental results show that the breakdown voltage for the positively stressed needle electrode (anode) is about 135% higher than that of the cathode (needle negative). This is abnormal compared with the common liquid, such as water. What is more, the time lag to breakdown is significantly shorter for negative breakdown. The integral images of the streamers in different polarities were recorded by the ultra-high-speed camera, as shown in Fig. 4. It can be obtained that the streamer is starting from the needle electrode under both the breakdown conditions. The positive streamer in PC is filamentary and dense like an umbrella. Obviously there are many discharge channels reaching the plate electrode. For each discharge path, there are numerous small branches on it. With the increase of the longitudinal distance, filamentary streamers increasingly become sparse and dusky. On the other hand, the negative streamer in PC is less branched and sparse like a leafless treetop. It has a very obvious main discharge channel, where there is almost no change of the brightness in the whole process. Moreover, differing from the positive streamer, the closer to the plate electrode, the shorter the branch. Meanwhile, many visible microbubbles appear near the plate electrode and the head of branches, which suggests that the bubbles should be responsible for the liquid breakdown. Comparing the streamer structure and the intensity of the light emission, we can draw a conclusion that the positive streamer has more energy than the negative one. The energy release for the negative streamer is more concentrated, but it is more dispersed for the positive streamer.
cpl-34-7-077701-fig3.png
Fig. 3. Two-dimensional Weibull distribution of breakdown voltages in different polarities.
cpl-34-7-077701-fig4.png
Fig. 4. Streamer images in different polarities. The needle electrode is replaced by arrows, and the rectangle replaces the plate electrode. (a) The positive streamer image in PC. (b) The negative streamer image in PC, in which microbubbles are marked with white circles.
The same experiment was carried out using two spherical electrodes made of brass, which formed a quasi-uniform field between electrodes. To make the breakdown occur more easily, the gap was adjusted to 1 mm, and the typical streamer images of PC in a quasi-uniform field are shown in Fig. 5. The results indicate that the proportion of the breakdown occurring from the cathode is higher, accounting for about 60% when repeating experiments 200 times. In terms of the positive streamer, it made up 15%. This is consistent with the above experimental results that the positive breakdown voltage is higher than the negative one. It is found that the breakdown may occur from the internal liquid, especially PC has a short standing time or insufficient mixing before it being carried out again. It could be assumed that there is a definite link between the phenomenon and the generation of microbubbles after breakdown.
cpl-34-7-077701-fig5.png
Fig. 5. Typical streamer images of PC in a quasi-uniform field. (a) The streamer is produced from the cathode. (b) The streamer is produced from the anode. (c) The streamer is produced from the internal liquid.
At present, there are a few reports on polarity effects of PC. Wang et al. found that the positive breakdown voltage of PC is always higher than the negative one, which was carried out by using the needle-plate electrodes made of brass with gaps of 0.5, 1.0 and 2.0 mm, and they argued that the electrode gap and whether it can generate enormous atomic oxygen anions when electrons move in bulk liquid have an important influence on the polarity effects of PC.[20] They paid more attention to the electric process in bulk liquid. In this work, the electrical conditions at the interface between the metal electrode and PC need to be focused to explore the mechanism of abnormal polarity effects of PC. The electrode condition always affects the electron transfer at the metal electrode-liquid interface and further affects the whole breakdown process. Lewis gave a detailed structure of the possible interface between the metal electrode and the liquid, which is a reasonable explanation of the polarity effect about non-polar insulating liquids.[21-23] For the polar liquid, such as PC, the polarity of liquid molecules greatly weakens the Coulomb attractive force between ions and electrons.[24] Dissociation does not produce a net charge in the liquid, but it is quite possible that the localized directional movement of the charge produces such a structure at the interface, as shown in Fig. 6. It is most likely that positive charges appear on the surface of the liquid side due to the difference in the Fermi level, which will produce the contact potential between the metal ion and the strongly polar molecule. Without applying a bias voltage, the model assumes that electrons moving around the positive metal cores form an electron cloud inside the electrode surface (a). Then positive ions and molecules from the liquid will chemically and physically adsorb on the electrode surface to form a so-called inner Helmholtz layer (IHL) due to electrostatic induction (b). Since the molecule is polar, the layer can be highly structured and has properties that are completely different from those of the bulk liquid. In the outer Helmholtz layer (OHL), the polar molecules will start to interact with the ions, forming a polarized sheath at the ionic surface to reach equilibrium (c). There is an obvious difference in the electronic energy states between IHL and OHL due to their above different microstructures. The outermost is the Gouy–Chapman diffusion space-charge layer, where the net charge decays to zero away from the electrode, and a stable polarized sheath at the ionic surface has been established (d). Between these layers, a series of charge movements are spontaneously undergone such as charge injection into the liquid or charge movement towards electrodes through the interface. The majority of the charge potential drop across the layer structure will be limited to the inner layer.
cpl-34-7-077701-fig6.png
Fig. 6. The interface structure between the metal electrode and the dielectric of PC. (a) Outer electron cloud. (b) Inner Helmholtz monolayer of ions and molecules (IHL). (c) Outer Helmholtz layer. (d) Gouy–Chapman diffuse space-charge layer.
Without applying a bias voltage, a net positive charge on the PC side produces contact potentials at each electrode interface (i), as shown in Fig. 7. When a stable bias voltage is applied, the layer structure will be strengthened or attenuated at different electrodes. For the anode, the electrons are attracted by the electrodes with a high mobility and the positive ions are excluded. It is conceivable that a large number of the negative charge are accumulated around the anode so that the electric field of the double layer is weakened (ii). If the bias voltage is strong enough, the positive charge at the interface will be replaced by the negative charge, resulting in potential sign reversal of the double layer (iii). Then, as the bias voltage is further increased, the strength of the double layer will continuously increase (iv). However at the cathode, electrons and negative ions will be driven away and the positive ions will gather around it, which strengthens the double Layer structure and the electric field already there. Considering the actual situation, the electrode surface cannot be very smooth and its micro-protrusion is prone to field-induced electron emission, especially under a high electric field. At the same time, the enhancement of charge layers in the cathode surface effectively reduces the interfacial tension resulting in the generation of sub-microscopic cavities (bubbles). Therefore, it is not difficult to draw a conclusion that the breakdown voltage of the positive streamer is higher than that of the negative one. In fact, the electrode parameters such as material or geometry, play an important role in the breakdown process which is an objective existence. However, the model with universality and objectivity still has guiding significance to explain the breakdown mechanism.
cpl-34-7-077701-fig7.png
Fig. 7. Feasible potential distributions between electrodes as a function of external voltage $V_{\rm b}$. (i) When $V_{\rm b}$ is zero, a net positive charge on the PC side produces contact potentials at each electrode interface. (ii) When $V_{\rm b}$ is increased, the electric field on the anode surface is reduced to zero while the cathode electric field is enhanced. (iii) As $V_{\rm b}$ further increases, a net negative charge is produced at the anode and the cathode field goes on increasing. (iv) Strong $V_{\rm b}$ causes considerable increase in fields at the anode and particularly at the cathode. The reduction of interfacial tension at the cathode now makes cavitation (bubbles) more likely to occur.
The morphological differences of streamers need to pay attention to the complex physical processes of charge movement in the bulk liquid. Under the influence of bias voltage, the head of the positive streamer ionizes and produces a large amount of electrons. These electrons can only return to the filamentary discharge area due to the electric field effect. However, there are many ions in this area, which were generated before there. These ions are trapped in the discharge region for a long time because of the low mobility and the collapse of the local electric field. Therefore, the recombination radiation probability between electrons and ions is very large in this area. At the same time, the photoelectron produced by recombination radiation can further cause the photoelectric ionization, which will aggravate the discharge process. In the negative streamer, electrons produced at the head have been dispersed constantly and away from the head, which lead to very weak recombination radiation. Therefore, the positive streamer generally has more branches, and the negative streamer usually has an obvious 'root'. In summary, by carrying out the experimental research on the polarity effect of streamer discharge in PC, we have found that the positive impulse breakdown voltage of PC is about 135% higher than the negative one. The positive streamer of PC is filamentary, but the negative streamer is less branched. To make clear this abnormal polarity effect of PC, a layer structure model is proposed, which builds at the interface between the metal electrode and PC. Applying a bias voltage will change the electrical condition of the interface resulting in the polarity effect. In detail, the external electric field will strengthen the double layer structure at the cathode but weaken it at the anode. The reduction of the cohesive property of the liquid at the cathode makes the cavitation more likely to occur. Thus the negative streamer is easier to produce. It has been observed that recombination radiation has a significant effect on streamer propagation. In the different polarities of the needle electrode, the strength of recombination radiation is different, and photoionization also plays an important role in the whole discharge process. The experimental results offer a comparison of electrical breakdown characteristics in different polarities, which will further promote the practical process of PC in the pulsed power technology. References Compact Rep-Rate GW Pulsed Generator Based on Forming Line With Built-In High-Coupling TransformerEffect of BaTiO 3 nano-particles on breakdown performance of propylene carbonateImpulse-driven Surface Breakdown Data: A Weibull Statistical AnalysisSignificantly enhanced impulse breakdown performances of propylene carbonate modified by TiO2 nano-particlesBubble Motion in Transformer Oil under Non-Uniform Electric FieldsEnhancement of Breakdown Voltage in AlGaN/GaN High Electron Mobility Transistors Using Double Buried p-Type LayersEnhanced dielectric breakdown performances of propylene carbonate modified by nano-particles under microsecond pulsesTheoretical study of the water pentamerBreakdown and prebreakdown phenomena in liquidsOptical and Pressure Diagnostics of 4-MV Water Switches in the Z-20 Test FacilityAnalysis of polarity effects in the electrical breakdown of liquidsThe influence of pressure and conductivity on the pulsed breakdown of waterWater as an insulator in pulsed facilities (Review)Microscopic analysis for water stressed by high electric fields in the prebreakdown regimeMeasurement and control for a repetitive nanosecond-pulse breakdown experiment in polymer filmsImpulse breakdown of water with different conductivitiesPolarity Effects of Propylene Carbonate on Breakdown Strength in Microsecond RangeBreakdown initiating mechanisms at electrode interfaces in liquidsBasic electrical processes in dielectric liquidsThe 1984 H. Tropper Memorial Lecture Electronic Ppocesses in Dielectric Liquids Under Incipient Breakdown StressSignificantly improved breakdown performances of propylene carbonate-based nano-fluids
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